I know some stuff about Doom speedrunning, but I never actually made a speedrun. I wanted to try a simple one to start, like an UV speed of a single level, but it doesn't seem to be a category here, so, where should I publish it?

Go into google and search something along the lines of "speedrun.com ultimate doom (level you want to run)" then just choose a catagory and speedrun ot

Speedrun com isn’t really best place for doom (unless you run new bethesda port - search unity ports for that). If you want to run doom - go to dsdarchive. Submissions are done via doomworld speeddemos section.